Kairus

I would assume it's possible, as people have been adding mega evolutions to pokemon that don't have it, Like Giratina allowing it to evolve into origin forme, the only thing that wouldn't be on the XY roms is the models for the megas, But im sure its possible to inject the ORAS mega models into XY including the skeletons and such, TTT's Sapphire randomiser has a Mega Milotic in it with a custom model, however a lot of the models are messed up in that but you would have ones made by nintendo from the ORAS rom I have little knowledge in Rom hacking atm, i can only manage to modify music in the games,